In a small saucepan over medium heat, simmer 1/2 cup water and sugar, whisking until mixture turns clear, about 1 to 2 minutes. Remove from heat, stir in mint, and allow to cool for 15 minutes; strain and chill until ready to serve. In a pitcher or bowl, combine syrup, vodka and juice. Divide ice and cocktail among 8 glasses. Top each with 2 tbsp Prosecco and serve.